Rack fixing methods: welding vs. bolting vs. bracketing
The rack transmits forces, not just motion. Every tooth that meshes discharges a tangential component and a radial one onto the fixing. Choosing the wrong method doesn't produce an immediate failure: it produces vibration, progressive backlash and early wear that shows up weeks after installation.
The methods compared
The rack converts the rotary motion of the motor into linear displacement of the supporting structure. The fixing must withstand this conversion over time, under all load conditions.
For nylon racks and nylon racks with a steel core, the main methods are bolting and bracketing. The choice depends on how the force transfers from the pinion, on the number of fixing points, on the length and thickness of the profile, and on the ability to intervene for maintenance.
| Method | Resistance to forces | Assembly and precision | Costs and maintenance | Preferential use |
|---|---|---|---|---|
| Bolting | Good if screws and holes are correctly sized | Adjustable with measuring instruments | Medium cost, simple maintenance | Automation with adjustment and replacement |
| Bracketing | Medium, depends on the spacing between brackets | Fast, less precise if brackets are elastic | Contained cost, attention to loosening | Light structures, temporary installations |
Reaction forces and pressure angle
The pinion meshes with the rack generating two forces: a tangential one, which moves the supporting structure, and a radial one, which pushes or pulls the components away from the fixing point. It's this second force that often determines the durability of the fixing over time.
With a 20° pressure angle, the radial force is about 36% of the tangential force. A tangential force of 1000 N generates about 364 N of radial force. Screws, brackets or welds must resist both tension and transverse thrust.
The tangential force is calculated from the motor torque and the pinion diameter:
Ft = 2T / d
Where T is the torque and d the pitch diameter of the pinion. Smaller pinions increase the transmitted force and the stresses on the fixing.
Bolting
The most versatile choice: it allows fixing, adjustment, pitch correction and replacement of sections without disassembling everything. You need a drill, level, pencil, measuring tape, screwdriver or driver, screws and suitable anchors.
For wall supports, the type of anchor depends on the load and the wall structure: for hollow walls toggle anchors are used, for solid masonry and heavy loads certified-quality nylon anchors.
Bracketing
Useful when you can't work along the entire base of the supporting structure, or when a fast assembly is needed. The brackets distribute the load over several points, but the spacing between them is decisive.
Brackets too far apart cause local bending of the rack, vibration, irregular wear and loss of contact with the pinion. Calculate the bracket spacing based on the radial force and the allowable bending for the chosen profile.
Thermal expansion and joints between sections
Nylon racks have a thermal expansion coefficient of about 80-100 × 10⁻⁶ /°C. On 1 meter with a variation of 50 °C, the elongation is about 4.5 mm: enough to change the backlash with the pinion, with a consequent increase in friction and wear.
To manage expansion, provide one fixed point and other points that allow longitudinal sliding. Not every fixing point has to be rigid. Slotted holes, assembly clearances, wide washers and controlled joints allow expansion without excessive stress.
The joints between sections can be made with plates, male-female inserts or overlaps. Maintain pitch continuity from one section to the next: any discontinuity becomes a mechanical obstacle to the passage of the pinion.
In nylon with a steel core, the differential expansion between plastic and metal can create internal stress on long runs or with strong thermal swings. Avoid excessive constraints and leave sliding margin at the non-fixed points.
Assembly notes
Make sure the supporting structure is completely mounted and secured before proceeding with the rack, in compliance with UNI EN 13241:2016. Use a measuring tape, level, drill and screwdriver to guarantee precision and alignment.
Operating checklist
- Verify motor torque, pinion diameter and tangential force.
- Calculate the radial force from the pressure angle (20°).
- Define the number and position of the fixing points.
- Use a level, tape and pencil before drilling.
- Drill the holes with the correct bit for the support material.
- Choose screws, anchors, inserts or brackets based on the calculated load.
- Leave clearance for thermal expansion.
- Check the pitch between sections before commissioning.
- Verify manually the sliding before powering the motor and control unit.
- Check the absence of obstacles along the entire travel.
For configurations with off-standard loads, long runs or particular thermal conditions, write to us at the technical office: we verify the fixing method and the sizing of the critical points together.